The invention relates to a method for crimping a multifilament thread as well as to a device for crimping a multifilament thread.
When melt spinning synthetic multifilament threads, it is a common practice that prior to the winding process a crimp of the filament strands is impressed on the threads. Preferably, such crimps of the filament threads are produced according to the compression chamber concept. By means of a transport nozzle, the multifilament thread is pneumatically guided and blown into a compression chamber. For this purpose, the transport nozzle comprises a thread channel which is connected with a compressed air source to produce a compressed air stream. The thread is then guided via the compressed air stream into a compression chamber in which the thread is compressed to a thread plug. In the process, the filament strands of the thread are deposited in loops and bows on the surface of the thread plug and are compressed by means of the compressed air stream. Thereafter, the thread plug is uncoiled outside the compression chamber to form a crimped thread. For example, such a method and a device are known from EP 0 554 642.
In the known method and device, the formation of the thread plug is monitored, on the one hand, to avoid blowing the thread plug out of the compression chamber and, on the other hand, to avoid plugging the compression chamber. For monitoring purposes, the pressure of the compressed air stream is measured at the outlet of the transport nozzle. To keep the conditions during forming the thread plug as constant as possible, the actual value of the pressure measurement is compared with a target value or a set-point range. In the event that a permissible deviation between the actual value and the target value is determined, regulation of the thread plug speed is performed which is determined via a needle roll on the outlet side of the compression chamber.
Therefore, the known method and device provide the possibility to maintain predetermined target values of a pressure of the compressed air stream. However, it is not possible to measure a filling level of the compression chamber or a position of the thread plug, which could result in undesired effects, for example, that the thread plug is blown out. Furthermore, when product changes are made, adjustments of the compressed air stream are required depending on the respective titer of the thread, which inevitably change the target values and result in new reference values of the pressure of the compressed air stream.
In the known method and device, efforts have been made to measure the position of the thread plug by means of optical sensors to prevent such disadvantages. However, such optical measurement systems have only a limited field of application because high temperatures and a plurality of air-borne particles, such as preparation residues and dye particles, result in quick contamination of the crimping environment. Experience has shown that optical systems are completely unsuited for reliable operation in the environment of a compression chamber.